FIG. 1 shows the cross-section of a typical FED 10 assembly; Plates 12 and 15 represent the cathode and anode plates, respectively. A glass spacer frame 13 is bonded to the anode and cathode plates via a frit seal 14. A hole in the cathode provides pump down, which is achieved through the exhaust tube 11. This tube 11 also serves as a cavity for an evaporable getter (not shown).
The interior space 16 within the FED should be evacuated of gas through the exhaust tube 11 in order for the FED to work properly. As the interior space 16 is evacuated, the pressure on the interior walls of plates 12 and 15 is vastly reduced in comparison to the pressure on the exterior walls of these plates. FIG. 2 shows what may happen when the pressure in the interior space 16 decreases to the levels required for proper operation of the FED. Under the atmospheric pressure, both the anode and the cathode plates, 15 and 12, tend to bend inward because there is no pressure within the FED to oppose the pressure being applied from outside the FED. This results in destructive compression and tension forces being applied to the frit seal 14. The inner portions of the frit seal 14 are compressed, and the outer portions of the frit seal are placed in tension. It is easy to understand that the larger the glass span of plates 12 and 15, the greater the compressive and tension forces that may be applied to the frit 14. For a given glass thickness n and seal width w, there is a maximum force which can be applied without tearing the seal 14 and cracking the plates 12 and 15. As the pixel pitch decreases (higher resolution) and the viewing area gets larger (higher information content), the pressure exerted on the anode and cathode plates, due to atmospheric pressure, becomes very high and presents a challenge to the manufacture of large area FEDs with a gap between the plates 12 and 15.
Thicker glass or stronger materials can be used for plates 12 and 15, but they do not really provide a scalable solution. While the seal width w can also be increased to spread the load and improve shear resistance, the required width often is in conflict with FED applications that require small peripheral widths of the frit seal 14. In addition, the increase in frit seal width requires larger glass plates 12 and 15, but does not increase the actual viewing area of the FED. Other display technologies, such as liquid crystal displays, do not suffer from this drawback of increased seal width.
One method of reducing the stress on the frit seals 14 is to use internal spacers within the interior space 16. Spacers are essentially insulative structures that form a bridge between the cathode plate 12 and the anode plate 15 within the interior space 16. The spacers can be used to keep a constant separation between the cathode and anode plates across the dimensions of these plates. This approach allows for the use of thin glass plates, similar to those used in the LCD technology, for the cathode 12 and anode 15 plates.
The presence of these spacers within the interior space 16 means that there can be no pixels where there are spacers. Thus, the inclusion of internal spacers may affect display resolution. In addition, this approach may preclude the use of high voltage phosphors, considered to be the best fit for the FED technology, because of the chance that the spacers will provide a path for flashovers between the cathode and anode plates 12 and 15. Further, the high voltage operation necessitates a large space between the plates 12 and 15, which means a large spacer, thus resulting in spacer-created dark regions.
Another method for reducing frit seal stress is to use thick glass plates for the cathode and anode plates, 12 and 15, to compensate for the unbalanced pressure forces on the plates. This approach is presently being used for displays smaller than 3" in diagonal. Larger displays require a thicker glass but also an increase in the width w of the perimeter seal. This increase places limitations on the display's ability to be used in applications where border area is at a premium (for example, in avionics displays). In addition, the weight increase is likely to result in a non-competitive package, even when compared to conventional CRTs. Even the use of stronger materials, such as glass ceramics, may not remove the need for a wider frit seal in order to reduce the point shear force.
With reference to FIG. 3, a third method of reducing frit seal stress has been to use a 3-piece FED 10 assembly including a rear piece 17 in the shape of a shrunken funnel to reduce the stress on the seal 14 between the spacer frame 13 and the cathode plate 12. This approach may strengthen the assembly by providing a rear piece 17 that does not deflect under pressure as much as a flat plate. The addition of the rear piece 17, however, results in a non-flat panel, making the display more bulky.
The rear piece 17 may provide another benefit by providing a location for getter material in the FED. The operation of an FED may be highly dependent on the maintenance of a gas free vacuum between cathode and anode plates, 12 and 15. Getter material within the FED is useful in capturing gas that is inside the FED. The inclusion of a rear piece 17 results in the formation of an additional wall within the FED on which getter material may be located. The advantage of using a rear piece to house getter material, however, is counter balanced by the bulkiness of the overall FED with a shrunken funnel shaped rear piece.